Marijuana (MJ) is the most widely used illicit drug globally and a major public health problem, especially among young people. According to the National Survey on Drug Use and Health, the incidence of first-time MJ use in the United States was 3.47 million people in 2013, which represents an increase from previous years and equates to approximately 9,507 new users per day (https://nsduhweb.rti.org). Heavy MJ use is detrimental to psychosocial functioning and can have serious adverse effects on both mental and physical health. Clinical manifestations of chronic use include tolerance to the subjective and cardiovascular effects of MJ, withdrawal, cardio-respiratory complications, and cognitive impairment. The escalating use rates in the general population suggest that treatment demand will continue to grow during the foreseeable future. Although some progress has been made in developing treatment strategies, high relapse rates persist and withdrawal symptoms likely contribute to the difficulty of remaining abstinent. The mechanism of action of MJ on neurochemistry, especially in relation to reported withdrawal symptoms, is still not well-understood. This R21 addresses this knowledge gap. Previous studies of cerebral metabolites and cognitive functions suggest that chronic MJ abuse alters GABA and glutamate levels. Based on others' recent finding of CB1 receptors recovery during MJ abstinence and our study of the abstinence process, we assume GABA and glutamate may recover to ?normal? levels or remain at the MJ ?adapted? levels via fluctuation during MJ abstinence, which could lead to a number of withdrawal symptoms. We hypothesize that an imbalance of GABA and glutamate during MJ abstinence may be the metabolic source of the reported withdrawal symptoms and that fluctuations of GABA and glutamate may impact individuals' responsiveness to medications. We propose to measure GABA and glutamate levels longitudinally during abstinence using proton MRS spectral-editing techniques in conjunction with behavioral measures of withdrawal using a full battery of sleep/wake activity, behavioral, cognitive, and performance assessments; and then evaluate the relationship between these neurochemicals and withdrawal symptoms. Data from this study will provide an initial molecular basis for withdrawal symptoms via changes of GABA and glutamate in selected brain regions of MJ abusers during abstinence, which will further elucidate the nature of MJ addiction, its complications, and the difficulty of maintaining abstinence. A deeper understanding of the causal factors underlying withdrawal symptoms may lead to better treatment targets and thereby improve outcomes. A subsequent R01 will be proposed based on the data of this study to determine the relationship between observed withdrawal symptoms and changes in brain chemistry, and to evaluate its possible utility as a predictor of relapse. This project is high-impact, as our approach will lead to a better understanding of the addiction/withdrawal process and thus inform treatment strategies for this notoriously refractory condition.